1. Field of the Invention
This invention relates to the field of resonant cavity filters and more particularly to an absorptive resonant cavity with substantially constant resistive input impedance of a predetermined resistive value at all off resonant frequencies.
2. Description of the Prior Art
In communications systems, it is often desirable to provide filtering for the receivers and transmitters so that intermodulation and splatter can be reduced. Cavity resonators have been affectively used in such systems are filters since they are very high Q circuits which can be easily inserted in a line connecting a transmitter amplifier or a receiver amplifier with an antenna.
However, cavity resonators in the prior art do not have a uniform resistive input impedance for frequencies outside the bandpass of the filter. This causes instability for power amplifiers coupled to the resonator which can destroy the amplifier and which complicate amplifier design. The typical prior art solution to this problem for transmitter circuits is to insert a circulator, which is both expensive and bulky, between the power amplifier and the cavity resonator. The addition of the circulator also results in undesired signal losses and greater circuit complexity.